Technical Field
Embodiments of the invention relate generally to image acquisition. Particular embodiments relate to x-ray imaging systems used for mammography.
Discussion of Art
Generally, x-ray imaging systems expose an x-ray detector, e.g., gamma photon scintillator or film, to an x-ray source, via a target object that is to be imaged. Attenuation or dispersion of photons emitted from the x-ray source within the target object produces a variegated image at the x-ray detector. This image then can be processed to ascertain radiopacity at various regions of the target object. For example, in mammography, where breast tissue is imaged, a region of higher than average radiopacity is understood to indicate the presence of a potentially pre-cancerous or cancerous lesion.
In medical imaging, it is generally desirable to minimize the size and intensity of an x-ray source, especially when imaging radiation-sensitive tissues such as breast tissue. In particular, it is desirable to minimize the radiation exposure needed to identify and localize, in three dimensions, regions of high radiopacity that could indicate precancerous cells. To accomplish this, a moving x-ray source may be used to provide a low x-ray dose to the target tissue while also obtaining volumetric detector data for use in localizing regions of high radiopacity. A moving x-ray source, however, presents a potential problem of image distortion along the x-ray source direction of motion.
As mentioned, it is also desirable to identify radiopaque areas in three dimensions. Describing or displaying a three-dimensional structure from a sequence of planar images obtained from different perspectives is referred to as “tomosynthesis.” The quality of tomosynthesis solutions depends upon the quantity and quality of planar images and on the total angle covered by the planar image array.
Tomosynthesis solutions generally can be categorized as “sharp” (providing relatively high resolution and fidelity of location within three dimensions) or “fast” (providing real-time or near-real-time imaging). For some types of medical imaging, such as mammography, sharp or fast solutions are exclusive choices. Fast tomosynthesis involves continuous source motion during exposure, therefore reducing signal transfer at higher frequencies, and loss of information, which precludes obtaining optimally sharp images. The fuzziness of fast tomosynthesis can be mitigated to some extent by a moving x-ray detector, however the final travel distance required for the detector eventually affects the possible imaging area due to positioning constraints of the patient/organ relative to the x-ray detector.
With reference to positioning constraints, it is desirable in medical imaging generally, and especially in mammography, to minimize the size of the imaging equipment that must be juxtaposed to a patient's body. Reducing the size of imaging equipment present a problem of constraining x-ray source movement, which detracts from the clarity of tomosynthesis solutions for the reasons discussed above. Reducing the size of imaging equipment also can constrain x-ray detector movement, which also can detract from the clarity of tomosynthesis as further discussed below.
For continuous detector motion, the x-ray detector travel distance proper to compensate apparent source size is estimated from 1/10th to ⅕th of the tube linear distance. For example, for a typical prior art tube travel of 16 cm) (+/−7.5°), the resulting x-ray detector trajectory is >16 mm, which could impact breast positioning. For further tube travel (+/−12.5°→27 mm detector travel) it becomes fairly impractical to compress to cover the imaging apparatus field of view.
In view of the above, it is desirable to provide apparatus and methods for moving source mammography that mitigates image distortion and apparatus volume envelope. Such apparatus and methods might also be helpful toward volumetric x-ray imaging, generally.